Just saw on twitter that someone came up with a patch for 3gs users that are JB'd with spirit. You are supposed to pick this patch up at cydia and enables you to install iOS 4 and JB it. I haven't tried it yet, just wondering if anyone else has heard of it.
Spirit2Pwn: Hack Allows Users To Update And Jailbreak iPhone 3GS With iOS 4, Which Is Already Jailbroken Using Spirit - iPhone Hacks
Spirit2Pwn: Hack Allows Users To Update And Jailbreak iPhone 3GS With iOS 4, Which Is Already Jailbroken Using Spirit - iPhone Hacks